1. Field of the Invention
The present invention relates to a developing device that develops an electrostatic latent image carried on an image carrier by a two-component developer including toner and magnetic particles, a process cartridge that includes the developing device, an image forming apparatus that includes the developing device or the process cartridge, and a method of developing a latent image.
2. Description of the Related Art
Developing devices are employed in image forming apparatuses to develop a latent image into a visible tonner image. Some of those developing devices, i.e., two-component developing devices, employ a two-component developer that is a mixture of toner and magnetic particles (hereinafter, referred to as a “carrier”). The two-component developer is typically housed in a developer container. It is preferable that the toner density (developer density) in the developer container be uniform so as to keep the image quality uniform. To maintain the toner density in the developer container uniform, a developer agitating/conveying member that agitates the developer in the developer container is typically provided inside the developer container. The developer in the developer container is then conveyed and supplied to a developer carrier. A plurality of magnetic poles is arranged under the surface of the developer carrier so that the developer, which contains the magnetic particles, magnetically sticks to the surface of the developer carrier. The developer carrier conveys and supplies the developer to an electrostatic latent image on an image carrier. The electrostatic latent image is developed by the toner in the developer and converted into a visual image.
Two screws are typically used as the developer agitating/conveying member: a supplying/collecting screw and an agitating/conveying screw. The supplying/collecting screw and the agitating/conveying screw are arranged in parallel so that the conveying directions thereof are opposite to each other. The supplying/collecting screw supplies the developer in the developer container to the developer carrier. The supplying/collecting screw also collects residual developer, which is the developer that remains on the developer carrier without being conveyed to the image carrier, and delivers the residual developer to the agitating/conveying screw. The agitating/conveying screw conveys the residual developer to a conveying path of the agitating/conveying screw, and the developer is delivered to the supplying/collecting screw, while being mixed and agitated therewith.
In this manner, the developer is circulated while mixing and agitating the developer. However, if the toner and the carrier in the developer are not mixed and agitated sufficiently, a toner density fluctuation and defective charging of the developer occurs. Toner density fluctuation and defective charging leads to an image density fluctuation that in-turn leads to degradation of image quality. The degree of agitation required to prevent such disadvantageous depends on the amount of toner consumption relative to the developer. However, because of the recent trend toward high-speed and down-sizing of image forming apparatus, the amount of toner consumption has increased with respect to the quantity of the developer. Therefore, it has become necessary to increase the degree of agitation even further.
In nowadays, colorization has advanced in electrophotographic image forming apparatuses. To improve the productivity, it is effective to adopt a tandem type image forming apparatus that forms an image by using four cylindrical photosensitive bodies for each color of yellow, magenta, cyan, and black. In the tandem type image forming apparatus, four photosensitive bodies are arranged in the transverse direction, and image creating devices such as a charging device and an exposing device are provided with respect to each of the photosensitive bodies. A developing device is also provided with respect to each of the photosensitive bodies.
To downsize such a tandem type image forming apparatus, the space between adjacent photosensitive bodies needs to be narrowed, and to do so, the size of the developing device also needs to be downsized in the horizontal (transverse) direction. However, with the developing device in which two screws are arranged in the horizontal direction, reducing of size is limited.
In a developing device disclosed in Japanese Patent Application Laid-open No. H11-202627, two agitating/conveying screws are arranged in a vertical direction beside a developing roller, thereby reducing the size in the transverse direction. However, in a typical developing device, the toner is consumed by being adhered on the electrostatic latent image during developing. Accordingly, the residual developer whose toner density is low and the fresh developer, which comes from the developer container, whose toner density is high, are supplied to the developing carrier (roller) at the same time. Because the toner densities of the residual developer and the fresh developer differ, it leads to fluctuation of image density.
To solve such a problem, in a developing device disclosed in Japanese Patent Application Laid-open No. 2003-263012, two agitating/conveying screws are arranged in a vertical direction, and the two screws are separated by arranging a partition plate therebetween. The toner density is made uniform, by supplying the developer by one of the screws, and collecting the developer by the other screw. In this developing device, the toner density is made uniform because the developer is sufficiently mixed and agitated before being supplied to the developer carrier.
However, the amount of developer gradually increases along the shaft direction of the collecting screw. In other words, there is less developer on the upstream side and more developer on the downstream side with respect to the direction of movement of the developer when the developer is move by the collecting screw. Accordingly, overflow and co-rotation of the developer tend to occur at the downstream side. Moreover, in this developing device, a part of the developer conveyed by the supplying screw is supplied to the developer carrier in the way of being conveyed to the downstream side from the upstream side. Accordingly, the amount of developer decreases at the downstream side compared with that at the upstream side, and at the most downstream portion, the height of the uppermost surface of the developer will be lowered. Subsequently, a supply shortage, in other words, a decrease in the amount of developer supplied to the developer carrier at the portion, may occur.